Metallogels are an important class of supramolecular materials, whose intrinsic properties stem from the non-covalent interaction between the metallic entity (metal or metal ion), and organic linker (polymer or small organic molecules) resulting in a stable extended network with voluminous immobilization of solvent molecules within it. Most of the metallogelators result from non-covalent interactions, mainly hydrogen bonding apart from metal-ligand coordination. An important property of metallogels lies in their ability to conduct protons due to the inherent H-bonding. Therefore, these materials could be utilized as proton conducting solid electrolyte in Proton Exchange Membrane Fuel Cells (PEMFC) as it demands a material with ability to conduct protons at high temperature (>100° C.) in dry conditions apart from high mechanical stability. This would essentially perk up the resistance to fuel cell impurities, improve the electrode kinetics apart from resolving the flooding issues, normally encountered in Nafion® based PEMFCs.
An important alternative for High Temperature Proton Exchange Membrane Fuel Cells (HT-PEFMCs) are phosphoric acid doped-Polybenzimidazole (PBI) membranes having high proton conductivity at temperatures up to 200° C. However, one of the problems affecting its performance is the leaching of phosphoric acid at higher levels of doping which limit any further improvement in its conductivity.
In the light of the above, it is evident that any prospective material that can be used as a solid electrolyte in a proton exchange fuel cell, needs to satisfy two criteria viz., (1) to separate the anode and cathode components (mainly the reactant gases) (2) to conduct protons across it, thereby completing the external electrical circuit and making the fuel cell operational for production of electricity.
Therefore, there exists a need for an intrinsically conducting electrolyte which could not only fasten and selectively transport proton at high temperature (100-200° C.) under anhydrous conditions but also effectively separate the electrode materials and reactant gases for an optimal overall performance of the PEFMC's. In this perspective, thermally as well as chemically stable metallogels might offer a perfect platform for immobilizing the proton conducting units via their coordination to the metal centers. However, till date, there are limited examples of such metallogels employed as proton conductors, one being a CuA-Ox xerogel, which exhibits a protonic conductivity of 1.4×10-5 S cm-1 at 65° C. under anhydrous conditions. It is well known that the proton conductivity depends on the number and mobility of charge carriers (protons).
Among the known protogenic molecules, phosphoric acid derived phytic acid (inositol hexakisphosphate) contains 12 replaceable protons and is thus capable of easily coordinating to multivalent ions. Moreover each phytic acid (PA) molecule contains six 6 phosphate ester (H2PO4) groups, well known for its amphoteric nature that allows proton conduction without any assistance from external proton carriers.
Although, phytic acid metallic complexes are known in the art, however, these are used to inhibit Polygalacturonase activity in microorganism which causes pathogenicity and spoilage of fruits and vegetables during plant tissue infections (Carbohydrate Polymers. Volume 95. Issue 1, 5 Jun. 2013, Pages 167-170).
Article titled “Proton-Conducting Supramolecular Metallogels from the Lowest Molecular Weight Assembler Ligand: A Quote for Simplicity” by S Saha et al. published in Chemistry—A European Journal, 2013, 19 (29), pp 9562-9568 two novel multifunctional metallogels were readily prepared at room temperature by simple mixing of stock solutions of CuII acetate monohydrate or CuII perchlorate hexahydrate and oxalic acid dihydrate.
Article titled “Proton conductivity enhancement by nanostructural control of poly(benzimidazole)-phosphoric acid adduct” by J Weber et al. published in Advanced materials, 2008, 20 (13), pp 2595-2598 reports mesoporous polybenzimidazole doped with phosphoric acid which shows enhanced proton conductivity compared to an equivalent, nonporous membrane. The introduction of a defined nanostructure into cross-linked poly(benzimidazole)/phosphoric acid composites is thus a promising approach towards membranes of high temperature stability suitable for fuel cell applications.
CN104022301A discloses a polymer—supported metal-organic framework materials phytic acid composite membrane preparation method and application. The membrane material prepared through the preparation method is good in proton conduction property even under low humidity.
Article titled “Enhanced proton conductivity of nafion hybrid membrane under different humidities by incorporating metal-organic frameworks with high phytic acid loading” by Z Li et al. published in ACS Appl Mater Interfaces, 2014; 6 (12), pp 9799-9807 reports Nafion/phytic@MIL hybrid membranes showed high proton conductivity at different RHs. In this study, phytic acid (myo-inositol hexaphosphonic acid) was first immobilized by MIL101 via vacuum-assisted impregnation method. The obtained phytic@MIL101 was then utilized as a novel filler to incorporate into Nafion to fabricate hybrid proton exchange membrane for application in PEMFC under different relative humidities (RHs), especially under low RHs.
In the light of the foregoing, there is an unmet need in the art to develop an intrinsically conducting electrolyte which could not only fasten and selectively transport proton at high temperature (100-200° C.) under anhydrous conditions but also effectively separate the electrode materials and reactant gases, for an optimal overall performance of the PEFMC's. Accordingly, the inventors of present invention had developed a ferric nitrate-phytic acid (FNPA) metallogel for use as conducting electrolyte in PEFMCs.